This invention relates to a method of detecting the reaction endpoint in an interfacial aromatic polycarbonate polymerization reaction.
Organic polycarbonate polymers are useful plastics especially known for their excellent engineering properties and their inherent flame resistance. Organic polycarbonate polymers can be prepared directly by the reaction of phosgene with an organic bishydroxy compound, such as the aromatic bishydroxy compound 2,2-bis(4-hydroxyphenyl)propane, commonly referred to as bisphenol A. Alternatively, organic polycarbonate polymers can be prepared by polymerization of organic chloroformate oligomer mixtures. These oligomers comprise bischloroformates, monohydroxy monochloroformates and bishydroxy compounds which can be represented by the general formulas I, II, and III respectively hereinbelow: ##STR1## wherein R is a divalent C.sub.6-30 aromatic radical and n is at least 1 and the number average for n is preferably less than 10. Low molecular weight (n.ltoreq.5) organic chloroformate oligomers are especially useful in the preparation of cyclic polycarbonate oligomers which may be converted to very high molecular weight polycarbonates.
For a variety of reasons it is desirable to know the stoichiometric endpoint of aromatic polycarbonate polymerization reactions such as those described hereinabove. In direct phosgenations to make capped polymer the use of too little phosgene results in unreacted hydroxyl endgroups. The resulting polymer possesses an undesirably low molecular weight, and the polymer's properties, especially impact strength, are compromised. At the other end, the use of too much phosgene results in wasted time and raw material, while the polymer produced may also be degraded. In the polymerization of chloroformate oligomers, dropping a batch too soon results in unreacted chloroformate or hydroxyl endgroups. The chloroformate tends to be corrosive, and the resulting polymer may possess an undesirably low molecular weight and impact strength. Dropping a batch too late also wastes time and degrades product. In some instances, a chloroformate polymerization reaction requires additional phosgene for completion, and it is desirable to know the quantity of phosgene needed.
Several methods are known for detecting the stoichiometric endpoint of an organic polycarbonate polymerization preparation. U.S. Pat. No. 4,814,420 discloses a method comprising monitoring the rate of heat generated by the polymerization reaction mixture per unit of phosgene consumed, until at the stoichiometric endpoint of the reaction a rapid rise in heat is detected. The rapid rise in heat is related to the hydrolysis of excess phosgene in the reaction mixture. Disadvantageously, this method detects excess phosgene, and therefore is responsive only after the actual endpoint of the reaction is achieved. Moreover, this method is limited to direct phosgenation reactions, and is not applicable to detecting the endpoint of chloroformate oligomer polymerizations.
U.S. Pat. No. 4,506,067 teaches a method for detecting the stoichiometric endpoint of an aromatic polycarbonate resin preparation comprising detecting an increase of phosgene gas occurring in the vapor phase portion of the reactor at or shortly after the stoichiometric endpoint of the reaction. Several detection methods are disclosed including passing the overhead vapors through a water stream and measuring the resulting pH. Since phosgene hydrolyzes to yield hydrochloric acid, the pH of the water stream drops at or shortly after the endpoint of the reaction. Alternatively, the overhead vapors can be monitored by infrared spectroscopy for a characteristic phosgene band at 850 cm.sup.-1. Disadvantageously, this method is limited to direct phosgenation reactions and cannot be applied to chloroformate oligomer polymerization preparations. Moreover, this method is based upon the presence of an excess of phosgene, and is therefore sensitive only after the actual endpoint of the polymerization reaction.
U.S. Pat. No. 4,378,454 discloses a method of preparing polycarbonates from dihydric phenols and phosgene in homogeneous mixtures of pyridine and a halogenated solvent. It is taught that periodically a sample of the reaction is mixed with an organic solution of 4-(p-nitrobenzyl)pyridine indicator to monitor the stoichiometric endpoint of the reaction. Just beyond the endpoint excess phosgene reacts with the indicator to give a color change from colorless to yellow. Disadvantageously, this method employs a sampling technique which is time consuming. More disadvantageously, this method is limited to direct phosgenation reactions and cannot be applied to chloroformate oligomer polymerization preparations. Even more disadvantageously, this method cannot be employed in two-phase interfacial reaction systems, such as those described in detail hereinafter, because the pyridine-containing solvent and indicator catalyze the hydrolysis of phosgene, thereby precluding reaction of phosgene with the indicator.
It would be desirable to have an improved and accurate method of detecting the reaction endpoint of a direct interfacial polycarbonate phosgenation polymerization reaction. It would be especially valuable if such a detection method could also be applied to detecting the reaction endpoint of interfacial chloroformate polycarbonate polymerization reactions.